THE SIR SUPPLIED DR. SPRANG .8G OF L-TEMET. HETEROTRIMERIC G-PROTEINS FUNCTION AS SIGNAL TRANDUCING MOLECULES IN MEDIATING THE PHYSIOLOGICAL RESPONSES ELICITED BY A VARIETY OF HORMONES. THE ALPHA SUBUNIT OF GI, A G-PROTEIN REGULATOR OF POTASSIUM CHANNELS AND ADENYLYL CYCLASE, EXHIBITS ITS ACTIVITY IN A GUANOSINE NUCLEOTIDE -DEPENDENT MANNER. THE MECHANISM OF THE GTP/GTP REGULATED CONFORMATION WHICH WILL BE INVESTIGATED BY X-RAY CRYSTALLOGRAPHIC STUDIES OF GIA. THE TELLUROMETHIONYL-GIA DERIVATIVE WILL AID IN THE DETERMINATION OF THE CRYSTALLOGRAPHIC PHASE PROBLEM BY ANOMALOUS SCATTERING METHODS. J. ODOM AND M. KUNKLE (U. S. CAROLINA) HAVE SUCCESSFULLY EXPRESSED TE-METHIONYL DIHYDROFOLATE REDUCTASE IN RECOMBINANT E. COLI (STRAIN MIC87, A METHIONINE AUXOTROPH). SUBSEQUENT CRYSTALLOGRAPHIC STUDIES DEMONSTRATE INCORPORATION AT INTERNAL SITES OF TE-MET. IN OUR OWN LABORATORY, WE HAVE EXPRESSED FULLY SUBSTITUTED SELENOMETHIONYL GIA SUBUNITS IN A SIMILAR SYSTEM. CRYSTALS OF SE-METGIA ARE ISOMORPHOUS WITH NATIVE CRYSTALS AND DIFFRACT AT COMPARABLE RESOLUTION. TE-MET WILL BE INCORPORATED INTO RECOMBINANT GIA USING PROCEDURES DEVELOPED BY DR. ODOM, ET AL. THE PROTEIN WILL BE PURIFIED AND CRYSTALLIZED. THE ANOMALOUS SCATTERING SIGNAL COMBINED WITH OTHER PHASE INFORMATION TO YIELD A THREE-DIMENSIONAL STRUCTURE.